1. Field of the Invention
The invention relates to a process for purifying the methanol employed in the preparation of formaldehyde.
2. Discussion of Prior Art
It is known that the starting materials required for the preparation of formaldehyde must be very pure, in order not to impair the activity and life of the silver catalyst to an excessive extent (compare, for example, (Auslegeschrift 2,323,758). The methanol or methanol/water mixture employed for the reaction, in addition to a number of organic substances, virtually always contains a number of inorganic compounds which, for example, promote side reactions which reduce the yield, or act as catalyst poisons. It is, therefore, necessary to purify the methanol before the oxidation and to use very pure water for the preparation of the methanol/water mixture or to subject the methanol/water mixture to subsequent purification.
Thus Auslegeschrift 1,235,881 discloses a process for the preparation of formaldehyde, in which the crude methanol is treated with alkali before, during and/or after removing a fraction boiling lower than methanol, and before the reaction to give formaldehyde. A similar method of purification is also disclosed in British 344,796. In accordance with this Patent, crude methanol, for example, containing traces of iron as an impurity, is likewise treated with alkalis. However, the purification with alkalis has the disadvantage that large quantities of strongly alkaline effluents are produced, the removal of which requires considerable technical outlay, to the detriment of the cost-efficiency of such purification processes.
Another possible means of purifying crude methanol consists in treatment with oxidizing agents. For example, purification of the crude methanol by treatment with chromic acid is disclosed in British No. 500,745, purification with permanganate and zinc chloride is disclosed in German No. 625,324 and purification by treatment with gas containing ozone is disclosed in Auslegeschrift No. 1,152,392. It is stated in German Auslegeschrift No. 1,231,677 to be particularly effective to purify crude methanol by treatment with oxygen or hydrogen peroxide in the presence of substances having a large surface area, such as silica gel, kieselguhr, alumina, earthy cerussite or active charcoal.
However, the purification processes mentioned have the disadvantage that effluent problems arise if chromic acid or permanganate are used. In addition, traces of chromic acid and permanganate, remaining in the treated methanol, can impair the activity of the sensitive silver catalyst. Nor does the use of ozone or hydrogen peroxide in treating the crude methanol bring any kind of advantage, since, on the one hand, these compounds are relatively expensive chemicals, which cannot be recovered after the treatment, and, on the other hand, only oxidisable substances can be removed thereby.
As can be seen from German Auslegeschrift No. 2,323,758, column 4, lines 14-29, in spite of purifying the crude methanol with the compounds mentioned above, the life of the silver catalyst, and also the yields and space/time yields of pure formaldehyde achieved with the catalyst, are still unsatisfactory, since some of the impurities contained in the crude methanol are not removed and deactivate the catalyst during the reaction.